This invention relates generally to apparatus for conveying and guiding webs, and more particularly, to fluidic bearings or air bars for conveying and guiding moving webs.
A variety of air-bearing apparatus for web conveyancing are generally well known in the art. U.S. Pat. No. 3,186,326 to Schmidt, U.S. Pat. No. 4,889,269 to Long et al., and U.S. Pat. No. 5,224,641 to Spicer teach exemplary air-bearing apparatus. The apparatus described in these patents support the web or media with a plurality of various holes or slot patterns. Through these orifices, pressurized fluid escapes under the web with sufficient normal force to float the web being conveyed. In addition, prior art devices have provided orifice configurations designed to provide various lateral center-guiding features, that is, for maintaining a moving web approximately centered on the air bar. The air bar apparatus described in the prior art tends to have problems when the web tension is removed, such as when web movement is stopped. When this occurs, the web is typically blown off the apparatus unless it is contained with another device. This may require re-threading of the web through or across the air-bar and other apparatus through which the web is being conveyed. This, in turn, results in lost operational time and increased material waste.
It is therefore an object of the present invention to provide an air-bearing apparatus for supporting the web in a non-contacting fashion.
It is a further object of the present invention to provide an air-bearing apparatus that controls web position radially relative to the air-bearing apparatus when the web is not under tension.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent to those skilled in the art upon a review of the specification, claims and drawings set forth herein. These features, objects and advantages are accomplished by providing preferably a single row of generally axially directed air jets around the centerline of the cylindrical surface of an air bar wherein adjacent jets are opposingly directed. The air jets exit the surface with an included angle of 25xc2x0 or less to the surface and should be approximately parallel with the axis of the cylindrical surface. The terms xe2x80x9cgenerally axiallyxe2x80x9d and xe2x80x9capproximately parallel with the axisxe2x80x9d as used herein are intended to mean that there is an axially directed component to the flow of air exiting the air jets, and that axially directed component is directed parallel to the axis of the air bar, or within an angle of plus or minus about 25xc2x0 of being parallel to the axis of the air bar. The air jets are preferably equally spaced around that portion of the circumference of the cylindrical surface that the web travels (the working circumference) and alternate to the left and right exiting the cylindrical surface at the web centerline.
Alternatively, two partial circumferential rows of axially directed air jets can be used. A first row of air jets would be generally directed toward one side of the air bar and a second row of air jets generally directed toward the opposite side of the air bar. The two rows of orifices should be staggered relative to one another. Additionally, it is believed that three or more rows of axially directed air jets can be used. For example, a first row could be positioned at the centerline with the second and third rows offset therefrom, one on each side of the center row. All three rows would be staggered from one another so that no two air jets are axially aligned with one another. It should be understood that the offset or spacing between rows of air jets may be very small such that two or more rows may simulate a single row.
A low pressure zone is created around each air jet by the high velocity air exiting the orifice. It is this low pressure which provides a vacuum interface that keeps the web from blowing off of the air bar while still supporting the web in a non-contacting manner of the cylindrical surface of the air bar. In other words, this interface between the web and the air jets holds the web down proximate to the cylindrical surface yet maintains approximately a 0.010xe2x80x3 air film between the web and the cylindrical surface. An additional feature of the present invention is its ability to self-thread or wrap the web such that, when combined with existing linear air tracks, the present invention can provide an entire self-threading web path. An exemplary air track or gas film conveyor for elongated strips of web material is taught in U.S. Pat. No. 5,209,387 to Long et al.
The air bar of the present invention has a self-wrapping feature. If the leading edge of the web is advanced to intercept the surface of the air bar tangentially, as the web is advanced further, the web is deflected (wrapped) around the air bar exiting where the apertures end.